Combustion And Emissions Characteristics Of Marine LNG Engine With Reformed Exhaust Gas Recirculation

Under the impetus of rules and regulations for the marine ship,the LNG fuel is becoming a primary alternative fuel to marine engines.Generally,the pure gas engines are lean-burn spark-ignition（SI）engines that operate on the Otto cycle for increasing thermal efficiency and resisting knocking and NO_xemissions.However,due to the high ignition energy and low flame speed of the natural gas,high air-fuel ratio may lead to the misfire,high exhaust temperature and the increase in the hydrocarbon（HC）and CO emissions.The reformed exhaust gas recirculation（REGR）technique can generate hydrogen-rich gas（reformate）via catalytic reforming exhaust gas and fuel,and then reformate is recirculated into the engine.With the combination of adding hydrogen and exhaust gas recirculation,the improvement in performance and emissions of the marine LNG engine can be achieved through the REGR technique.Since the exhaust-gas reformer is integrated with the LNG engine to realize reformed exhaust gas recirculation,there is a complex interaction between the reformer and engine,limiting the application of REGR technique on emissions control of the marine LNG engine.In the present study,the following studies had been carried out.（1）An experimental investigation into the effects of reformate addition ratio(R_re)and excess air ratio（λ）on the combustion and emissions characteristics of a marine NG engine under various loads was conducted.The addition of hydrogen-rich reformate improved the trade-off relationship between the NO_xemissions and brake specific fuel consumption（BSFC）of the marine NG engine,decreasing the NO_xemissions and BSFC simultaneously.It is demonstrated that the combination of REGR and the lean-burn combustion strategy could reduce the NO_xemissions to meet the IMO TierⅢNO_xemissions legislations and maintain relatively low BSFC.（2）A multi-dimensional computation fluid dynamics model coupled with a detailed chemical kinetic mechanism was developed to investigate the effects of reformate addition ratio(R_re)and exhaust valve closed（EVC）timing on the total emissions as well as the sources of HC and CO emissions from the engine.Results shown that the unburned CH₄in the cylinder was the main component of the total CH₄emissions.The oxidization of CH₄was promoted with the increase of R_reat high load,and therefore the total CH₄emissions decreased.However,the total CO emissions increased with the increase of R_re,and it is demonstrated that the unburned CO from the added reformate increased and turned to be the main sources of the total CO emissions.With the delay of EVC timing,total CH₄and CO emissions firstly remained nearly constant and then increased dramatically.（3）A platform of an exhaust gas-fuel reformer connected with the marine LNG engine was set up for generating on-board hydrogen.Results shown that reformate can be generated by reforming mixture of engine exhaust gas（about 400℃）and methane via reformer with Ni/Al₂O₃catalyst,and hydrogen concentration of reformate was between 6%and 12%by volume.Methane to oxygen ratio（M/O）andλaffected the components and temperature of reactant in the reformer,while reformed exhaust gas ratio(R_EG)changed the gas hour space velocity during exhaust gas-fuel reforming processes,resulting in the difference in the components of the reformate and thermal efficiency.At the present experimental condition,the highest H₂concentration reformate was generated under the M/O of 2.0,λof 1.55 and R_EGof6%.（4）A special designed exhaust reformer was integrated with the marine LNG engine and then the experimental study on the performance of the closed-loop system of the reformer and engine under different engine loads was conducted,exploring the emission reduction potential of REGR technique for the marine LNG engine.NO_xemission from the marine LNG engine with REGR was reduced by 60-70%at REGR rate of 4.7-5.8%and met the IMO TierⅢemission regulations under different loads.BSFC of the engine with REGR was higher than that of the prototype engine by1.1-2.9%,due to heat loss from reformer.（5）A comprehensive model of closed-loop system with engine and reformer was established and used to investigate the effects of air dilution and reformate dilution on the engine performance.The air dilution rate and reformate dilution rate were characterized byλand REGR rate,respectively,and then their influences on combustion characteristics,BSFC and emissions were explored and compared.In addition,normalized dual-dilution rate was used to evaluated the coupled relationship between air dilution and reformate dilution.Finally,the multi-objective optimization algorithm was employed to obtain the optimal dual-dilution parameters to realize the energy efficiency improvement and emission control of the marine LNG engine.